1. Field of the Invention
The present invention relates to check valves and back pressure valves, and more particularly to check valves for large conduits where the inlet and outlet openings of the valve are axially aligned and the valve member moves axially in relation to the valve seat.
2. Description of the Prior Art
A check valve in a conduit system is designed to prevent the inversion of the liquid flow, should such an inversion become possible through the failure of the pipe, or through pump failure, for example. In the event of such failure, the check valve has to immediately close the conduit, in response to the cessation of flow, or in response to an incipient reverse flow. Ideally, the operation of a check valve should follow the first of the two conditions, but such an operation requires the presence of a closing bias derived from a spring or from gravity. Such a closing bias, however, will then act as a throttling force against the liquid flowing in the conduit.
In the case of a check valve responding to an incipient reverse flow of the liquid, it is of crucial importance that the response be as rapid as possible. It is well established that a liquid column moving in a pipe under pressure, when subjected to a sudden deceleration, is capable of developing considerable deceleration forces, sometimes as high as several times the operating pressure. Pressure surges of this type are generally undesirable, as they result in uncontrolled stress conditions in the pipe system, including pipe failure under certain extreme conditions.
Known check valves for large conduits are generally of the type featuring an enlarged-diameter housing portion, at the entrance side of which is arranged a radially inwardly narrowed valve seat, a matching axially movable valve member being positioned inside the enlarged-diameter housing portion. In one prior art solution, disclosed in British Pat. No. 336,870, the valve member has the outline of a drop, being carried by a pair of parallel links which are pivoted radially outside the valve diameter, inside a laterally extending head. A counterweight attached to the parallel links applies a closing bias to the valve member. Another prior art valve, described in British Pat. No. 304,751, has a shallow, dish-shaped valve poppet which, in the open position, cooperates with the forward end of a stationary core body to form a drop-shaped centrally located core assembly. The valve poppet is guided for axial motion in relation to the core body, a compression spring urging the valve poppet toward its closed position.
A major shortcoming of the first-mentioned valve is the large inertia of its moving parts. The second prior art valve, though featuring a greatly reduced closing inertia, requires the action of a closing spring. Other prior art solutions have valve members moving along an arc into the open valve position, the latter being located laterally outside the liquid flow. Their closing action is too slow to prevent the establishment of a substantial reverse flow. The result is a sharp pressure surge, when the valve member is closed against its seat.
The requirements in size and response speed of check valves for large conduits have been steadily increasing, due to higher flow speeds and higher operating pressures in the conduits. A good example of this trend is found in the field of thermal power plants, especially atomic power plants. A pipe failure in the main feed pipe of such a power plant, for instance, if not checked by appropriate check valves, can lead to very severe secondary damage in the pipe system. As a result, it has become the practice in this field to specify check valves having a response time of only a few milliseconds.